JP7624186B2 - Sodium titanium phosphate and its uses - Google Patents

Description

The present disclosure relates to sodium titanium phosphate and its uses.

Sodium secondary batteries, which do not use lithium, a rare metal, not only have advantages in terms of resources and cost, but also have the potential to be applied to new materials and have excellent high-rate charge and discharge characteristics.
For sodium secondary batteries to date, those equipped with electrolytes similar to those used in current lithium secondary batteries, mainly non-aqueous electrolytes, have been considered.
Recently, it has been reported that the narrow potential window of 1.23 V resulting from the decomposition of water, which was the greatest weak point of aqueous batteries, can be expanded to over 2 V by increasing the concentration of the electrolyte (Non-Patent Document 1), raising expectations for the practical application of aqueous sodium secondary batteries.
In particular, for larger batteries, aqueous sodium secondary batteries using an aqueous electrolyte are suitable from the standpoints of cost and safety.
Regarding the negative electrode material, which is another issue for practical use of aqueous sodium secondary batteries, Nasicon-type NaTi ₂ (PO ₄ ) ₃ has been found to function, but its cycle stability is low and improvement of this is an issue (Non-Patent Document 2).

Ruben-Simon et al, ACS Energy Letters, 2, 2005 (2017) Sun Il Park et al, Journal of The Electrochemical Society, 158(10) A1067-A1070 (2011)

The present disclosure has an object to provide at least one of sodium titanium phosphate, and further, a negative electrode active material that provides a sodium secondary battery having superior output characteristics compared to conventional aqueous sodium secondary batteries, and a sodium secondary battery equipped with the same, and in particular, an object to provide a negative electrode active material that provides an aqueous sodium secondary battery having higher output characteristics compared to conventional aqueous sodium secondary batteries equipped with Nasicon-type _NaTi2 ( _PO4 ) ₃ as the negative electrode active material.

In the present disclosure, it has been discovered that a sodium titanium phosphate composition having a surface layer containing a sodium titanium phosphate of a different composition has superior output characteristics compared to conventionally known negative electrode materials for sodium secondary batteries, and that by using this in the negative electrode, a sodium secondary battery with superior output characteristics can be constructed.
That is, the present invention is as described in the claims, and the gist of the present disclosure is as follows.

[1] Sodium titanium phosphate represented by the general formula _{Na1+ xTi2} ( _PO4 ) ₃ (where 0≦x≦2), characterized in that it has a surface layer containing _Na5Ti ( _PO4 ) ₃ .
[2] The sodium titanium phosphate according to the above [1], wherein the crystal structure of the sodium titanium phosphate is a Nasicon type crystal structure.
[3] The sodium titanium phosphate according to the above [ ₁ ] or [2], wherein the surface layer is more than 0 mass% and less than 5 mass% of the sodium titanium phosphate represented by the general formula Na1 _{+xTi2 ( PO4} )3 (where 0≦x≦2).
[4] The sodium titanium phosphate according to any one of [1] to [3] above, wherein the thickness of the surface layer is greater than 0 nm and less than or equal to 50 nm.
[5] The sodium titanium phosphate according to any one of the above [1] to [4], having a crystallite size of 10 nm or more and 100 nm or less.
[6] A method for producing sodium titanium phosphate according to any one of the above [1] to [5], comprising the steps of calcining a mixture containing a sodium source, a phosphate source, and a titanium source at 150°C or more and 400°C or less to obtain a primary calcined product, crushing the primary calcined product, and then calcining the product at 400°C or more and 900°C or less.
[7] A negative electrode active material containing the sodium titanium phosphate according to any one of [1] to [5] above.
[8] A sodium secondary battery comprising a negative electrode containing the sodium titanium phosphate according to any one of [1] to [5] above, a positive electrode, and an electrolyte.
[9] The sodium secondary battery according to the above [8], wherein the electrolytic solution is an aqueous electrolytic solution.

According to the present disclosure, it is possible to provide at least one of sodium titanium phosphate, a negative electrode active material that provides a sodium secondary battery having superior output characteristics compared to conventional aqueous sodium secondary batteries, and a sodium secondary battery having the same. Furthermore, it is possible to provide a negative electrode active material that provides an aqueous sodium secondary battery having high output characteristics compared to conventional aqueous sodium secondary batteries having Nasicon type NaTi ₂ (PO ₄ ) ₃ as the negative electrode active material.

FIG. 1 shows XRD patterns of sodium transition metal phosphates of Examples 1 to 3 and Comparative Example (a: Comparative Example 1, b: Example 1, c: Example 2, d: Example 3, e: PDF pattern (ICDD: 33-1296 NaTi ₂ (PO ₄ ) ₃ ). FIG. 2 shows XRD patterns of NaTi ₂ (PO ₄ ) ₃ of Examples 5 and 6 (f: Example 5, g: Example 6, h: PDF pattern (Na ₅ Ti(PO ₄ ) ₃ ). FIG. 3A is a cross-sectional TEM image of the sodium transition metal phosphate of Example 2. FIG. 3B is a cross-sectional TEM image of the sodium transition metal phosphate of Comparative Example 1. FIG. 4 is a diagram showing the results of the charge-discharge cycle test of Examples 1 to 3 and Comparative Example 1 (a (▲): Example 1, b (■): Example 2, c (◆): Example 3, d (●): Comparative Example 1).

Below, the sodium titanium phosphate of the present disclosure is described with reference to an example embodiment.

<Sodium Titanium Phosphate>
The present embodiment is sodium titanium phosphate represented by the general formula _{Na1+ xTi2} ( _PO4 ) ₃ (where 0≦x≦2), characterized by having a surface layer containing _Na5Ti ( _PO4 ) ₃ . By having a surface layer containing _Na5Ti ( _PO4 ) ₃ , sodium ions (Na ⁺ ) can be easily diffused on the surface of sodium titanium phosphate (hereinafter also referred to as "NaTP salt").
The NaTP salt in this embodiment is a phosphate compound containing sodium and titanium, specifically a sodium titanium phosphate represented by the general formula _{Na1+xTi2 ( PO4} ) ₃ (where 0≦x≦2), and more preferably _NaTi2 ( _PO4 ) ₃ (i.e., x=0 in the general formula Na1 _+xTi2 ( _PO4 )). In this embodiment, the composition of the NaTP salt can be determined by a known composition analysis method, for example, inductively coupled plasma optical emission spectrometry ( _ICP analysis), atomic absorption spectrometry, TEM-EDS, or the like, and can be determined by ICP analysis using a general ICP optical emission spectrometer (ICPS-8100, manufactured by Shimadzu Corporation).
The NaTP salt of this embodiment preferably has a Nasicon-type crystal structure (so-called Nasicon-type NaTP salt). The Nasicon-type crystal structure can be identified by comparing the XRD pattern of the NaTP salt of this embodiment with the PDF pattern (ICDD: 33-1296 NaTi ₂ (PO ₄ ) ₃ ).

The NaTP salt of this embodiment has a surface layer (hereinafter, simply referred to as "surface layer") containing _Na5Ti ( _PO4 ) ₃ . Compared to sodium titanium phosphate expressed by the general formula _{Na1+xTi2 ( PO4} ) ₃ (where 0≦x≦2), the surface layer has a composition in which sodium is in excess, and this surface layer functions as a sodium buffer layer during high-power discharge, and as a result, it is believed to exhibit high power characteristics.
The surface layer may be present on at least a part of the NaTP salt without inhibiting the diffusion of sodium, and is preferably present on at least a part of the surface of the NaTP salt. The form in which the surface layer is present on at least a part of the NaTP salt includes at least the presence of the NaTP salt and Na ₅ Ti(PO ₄ ) ₃ via the interface between the two, that is, the surface layer forms an interface with the NaTP salt. As a result of the interface between the NaTP salt and Na ₅ Ti(PO ₄ ) ₃ , it is considered that the diffusion coefficient of sodium is equivalent to that of the NaTP salt without the surface layer.
The surface layer does not need to cover the entire surface of the NaTP salt, and may be in the form of islands in a sea. That is, the surface layer may be at least one of Na ₅ Ti(PO ₄ ) ₃ and a composition containing Na ₅ Ti(PO ₄ ) ₃ , preferably Na ₅ Ti(PO ₄ ) ₃ , present on at least a part of the surface of the NaTP salt.
The surface layer may contain Na ₅ Ti(PO ₄ ) ₃ , and may contain TiO ₂ in addition to Na ₅ Ti(PO ₄ ) _3. However, it is preferable that the surface layer is made of Na ₅ Ti(PO ₄ ) ₃ , and it is more preferable that the surface layer is made of Na ₅ Ti(PO ₄ ) ₃ having a Nasicon type crystal structure.
The NaTP salt of this embodiment may have a surface layer on the entire surface or at least a part of the surface, and may have a surface layer on at least a part of the surface. In addition, Na ₅ Ti(PO ₄ ) ₃ may be present on the entire surface or at least a part of the surface of the NaTP salt particle, preferably at least a part of the surface. The nature of Na ₅ Ti(PO ₄ ) ₃ contained in the surface layer is not particularly limited, and may be at least one of crystalline and amorphous, and further at least one of dense and porous. Furthermore, the surface layer preferably contains Na ₅ Ti(PO ₄ ) ₃ produced by the reaction of a sodium source, a phosphorus source, and a titanium source on the surface of the NaTP salt. This makes it easier to suppress polarization (hysteresis) during charge and discharge cycles. The presence of the surface layer, which is a coating film, can be confirmed by TEM observation (transmission electron microscope observation). In TEM observation, the surface layer is observed as a field image different from that of the NaTP salt, and the surface layer is observed as a field image having a darker tone than that of the NaTP salt.

Since sodium ions (Na ⁺ ) tend to diffuse easily, the ratio of the surface layer in this embodiment (hereinafter also referred to as the "coating amount") is preferably such that the lower limit exceeds 0% by mass, and is further 0.01% by mass or more, and further 0.1% by mass or more, relative to the NaTP salt. The upper limit is preferably 20% by mass or less, further 15% by mass or less, and further 5% by mass or less, relative to the NaTP salt. Any combination of these upper and lower limits is acceptable. Therefore, the ratio of the surface layer in this embodiment is preferably, for example, such that the ratio exceeds 0% by mass and is 20% by mass or less, and is further 0.01% by mass or more and 15% by mass or less, and further 0.1% by mass or more and 5% by mass or less, relative to the NaTP salt. The method for measuring the coating amount is arbitrary, and for example, quantitative analysis by TEM-EDS can be mentioned. If the coating amount exceeds 20% by mass, a surface layer that does not inhibit the diffusion of sodium is not formed, and the diffusion coefficient of sodium ions is significantly reduced.
The thickness of the surface layer is arbitrary, provided that it is thick enough not to impede the diffusion of sodium.
The thickness of the surface layer is preferably such that the lower limit exceeds 0 nm, or is 0.1 nm or more. The upper limit is preferably 50 nm or less, or is 40 nm or less, or is 5 nm or less. Any combination of these upper and lower limits is acceptable. Therefore, the thickness of the surface layer in this embodiment is preferably, for example, more than 0 nm and 50 nm or less, or is 0.1 nm or more and 5 nm or less. The thickness of the surface layer can be determined by TEM observation, and the thickness of the surface layer observed in the TEM observation diagram (i.e., the surface layer can be confirmed in the TEM observation diagram, and the maximum thickness of the surface layer observed in the TEM observation diagram) is more than 0 nm or 0.1 nm or more, and is 50 nm or less, 40 nm or less, 5 nm or less, or 3 nm or less.
The crystallite size of the NaTP salt of this embodiment is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. The upper limit is preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 40 nm or less. Any combination of these upper and lower limits is acceptable. Therefore, the crystallite size of the NaTP salt of this embodiment is preferably, for example, 10 nm or more and 100 nm or less, or 20 nm or more and 50 nm or less, or even 20 nm or more and 40 nm or less, or even 30 nm or more and 40 nm or less. By having the crystallite size in this range, the NaTP salt of this embodiment is more likely to efficiently intercalate and deintercalate sodium, and is expected to have high output characteristics as a negative electrode active material for a sodium secondary battery.
In this embodiment, the "crystallite size" is the _size (hereinafter also referred to as _{"WH size} ") obtained by the Williamson _- Hall method from two or more XRD peaks belonging to the XRD pattern of sodium titanium phosphate represented by the general formula Na _{1 + x} Ti 2 (PO ₄ ) ₃ (where 0≦x≦2). Specifically, the following plots are made for two or more XRD peaks that can be attributed to the general formula Na 1 + x Ti ₂ (PO 4 ) 3 (where 0≦x≦2), preferably all XRD peaks that can be attributed to the general formula Na _{1 + x} Ti ₂ (PO ₄ ) ₃ (where 0≦x≦2). The following linear approximation formula is obtained by the least squares method of the obtained plots of multiple points, and the reciprocal of the y-intercept of the linear approximation formula is the crystallite size.
＜Plot＞
Y=(β・sinθ)/λ
X = sin θ / λ
<First-order approximation formula>
Y=2η・X+(1/ε) ...(1 formula)
In these formulas, β is half-width (°), θ is diffraction angle (°), λ is the wavelength of radiation source (nm), η is non-uniform distortion, and ε is crystallite diameter (Å), and 1/ε in the first-order approximation formula is y-intercept. In addition, in the present embodiment, there is no particular limitation on the XRD peak used for calculating WH diameter, and for example, all XRD peaks that belong to NaTP may be used.
The NaTP salt of this embodiment exhibits high output characteristics when used as a negative electrode active material in a sodium secondary battery. The configuration of the sodium secondary battery and the conditions of the charge/discharge test are described below.
(Sodium secondary battery)
Electrolyte: 17 mol/kg _NaClO4 aqueous solution Negative electrode: 70% by mass of NaTP salt, 25% by mass of acetylene black, and
Negative electrode consisting of 5% by mass of polytetrafluoroethylene Positive electrode: 70% by mass of Na ₂ Ni[Fe(CN) ₆ ], 25% by mass of acetylene black, and
Positive electrode made of 5% by mass of polytetrafluoroethylene (charge/discharge test conditions)
Current density: (1 to 5 cycles) 2mA/ ^cm2
(6 to 10 cycles) 5mA/ ^cm2
(11 to 15 cycles) 10mA/ ^cm2
(16 to 20 cycles) 15mA/ ^cm2
(21 to 25 cycles) 20mA/ ^cm2
(25 to 30 cycles) 25mA/ ^cm2
(31 to 35 cycles) 2mA/ ^cm2
Voltage: -0.9V to -0.3V (vs Ag/AgCl reference electrode)
Charge/discharge temperature: 25°C

<Method of producing NaTP salt having a surface layer>
The method for producing the NaTP salt of this embodiment is arbitrary, but may include a production method having a step of calcining a mixture containing a sodium source, a phosphate source, and a titanium source at 150° C. to 400° C. to obtain a primary calcined product, which is then crushed and calcined at 400° C. to 900° C. (hereinafter also referred to as the “calcination step”). This makes it possible to obtain a NaTP salt having a surface layer containing Na ₅ Ti(PO ₄ ) ₃ .
The mixture subjected to the firing step is a composition containing a sodium source, a phosphate source, and a titanium source (hereinafter, these are also collectively referred to as "precursors").
The sodium source may be any compound containing sodium (Na), and is preferably one or more selected from the group consisting of sodium carbonate, sodium hydroxide, and sodium chloride, more preferably at least one of sodium carbonate and sodium hydroxide, and even more preferably sodium carbonate.
The phosphate source may be any compound containing phosphoric acid (PO ₄ ), and is preferably one or more selected from the group consisting of pyrophosphoric acid, polyphosphoric acid, and ammonium dihydrogen phosphate, and more preferably ammonium dihydrogen phosphate.
The titanium source may be any compound containing titanium (Ti), and may be at least one of titanium oxide and organic titanium compound. A preferred titanium oxide is titania (TiO ₂ ), and a preferred organic titanium compound is one or more selected from the group consisting of titanium-containing alkoxide, acylate and chelate compounds, titanium-containing alkoxide, or titanium tetrabutoxide (Ti(OCH ₂ CH ₂ CH ₂ CH ₃ ) ₄ ). A preferred titanium source is titanium tetrabutoxide.
Particularly preferred precursors include sodium carbonate, diammonium hydrogen phosphate and titanium tetrabutoxide.
The composition of the mixture is preferably such that sodium is in excess relative to the composition of the NaTP salt, particularly relative to the sodium titanium phosphate represented by the general formula _{Na1+xTi2 ( PO4} ) ₃ (where 0≦x≦2). For example, the ratio of Na [mol] to Ti [mol] in the mixed composition is more than 1.0 or 1.1 or more, and is 3 or less, 2 or less, or 1.5 or less. It is believed that by having such a composition, a surface layer is formed on the surface of the particles of the NaTP salt through the preliminary firing and main firing described below.
Such a mixture can be obtained by mixing precursors so as to have a composition in which sodium is excessive with respect to the target sodium titanium phosphate represented by the general formula _{Na1+xTi2 ( PO4} ) ₃ (where 0≦x≦2). For example, when producing a NaTP salt represented by _NaTi2 ( _PO4 ) ₃ , the sodium source and the titanium source can be mixed in such a way that the ratio of Na[mol] to Ti[mol] is more than 0.5 or 0.55 or more and is 1.5 or less, 1 or less, or 0.75 or less; and when producing a NaTP salt represented by _Na2Ti2 ( _PO4 ) ₃ , the sodium source and the titanium source can be mixed in such a way that the ratio of Na[mol] to Ti[mol] is more than 1.0 or _1.1 or more and is 3 or less, 2 or less, or 1.5 or less.
The mixture is obtained by mixing the precursors uniformly. The mixing method can be at least one of wet mixing and dry mixing, and further wet mixing. A particularly preferred mixing method is a method of drying a mixed solution containing the precursors and a solvent after stirring or while stirring. The solvent can be at least one of water and alcohol, and further water. The stirring can be performed under conditions that allow the precursors to be mixed uniformly, and can be exemplified by stirring the mixed solution at a stirring speed of 100 to 500 rpm, and the stirring speed can be increased as the amount of the mixed solution increases. The drying can be performed under conditions that allow the solvent to be removed from the mixed solution, and can be exemplified by air, at 60°C or higher or 70°C or higher, and 150°C or lower, 120°C or lower, or 100°C or lower. This allows the mixture to be obtained in which the precursors are mixed in a uniform state.
In the manufacturing method of this embodiment, the mixture is fired at 150° C. or more and 400° C. or less. As a method of firing to obtain a primary fired product (hereinafter also referred to as "pre-firing"), firing in an oxidizing atmosphere, preferably in air, at 150° C. or more, 250° C. or more, or 300° C. or more and 400° C. or less, less than 400° C., or 370° C. or less can be mentioned. The time of pre-firing is arbitrary, but can be 30 minutes or more and 10 hours or less. It is considered that the sodium concentration becomes a gradient composition between the surface and the inside of the particles of the mixture, and the sodium concentration is higher on the particle surface than in the particle inside by the pre-firing.
After the preliminary firing, the resulting primary fired product is crushed by any method that can remove loose agglomerates of the primary fired product, such as dry grinding using a mortar.
In the production method of this embodiment, the primary fired product after crushing is fired at 400° C. or more and 900° C. or less (hereinafter also referred to as "main firing"). Examples of the main firing method include firing in an oxidizing atmosphere, preferably in the air, at a temperature of 400° C. or more, more than 400° C. or more, or 600° C. or more, and 900° C. or less, 800° C. or less, or 750° C. or less. The time for main firing is arbitrary, but examples include 30 minutes to 10 hours. By main firing, NaTP salt having a surface layer is produced from a mixture having a composition with a gradient sodium concentration.

<Negative Electrode Active Material>
Next, a negative electrode active material containing the NaTP salt of the present disclosure will be described with reference to an example of an embodiment.
In this embodiment, the "negative electrode active material" refers to an electrode active material of an electrode having a lower potential among the electrodes constituting an electrochemical device, and in particular, an electrode active material of the negative electrode of a sodium secondary battery.
The negative electrode active material of this embodiment may contain the NaTP salt of this embodiment (i.e., the NaTP salt having a surface layer) or may consist of only the NaTP salt of this embodiment (i.e., only the NaTP salt having a surface layer). On the other hand, the negative electrode active material of this embodiment may contain an active material other than the NaTP salt of this embodiment, such as an active material such as a sodium transition metal compound.
The negative electrode active material of this embodiment may have a coating layer, preferably a coating layer having electrical conductivity (i.e., a conductive layer), on a part or the whole of the surface of the negative electrode active material, such as a carbon layer. In this case, the coating layer may be on a surface layer containing _Na5Ti ( _PO4 ) ₃ . However, the coating layer may be present on the surface of the NaTP salt.

<Sodium secondary battery>
Next, a sodium secondary battery characterized by including a negative electrode containing the NaTP salt of the present disclosure, a positive electrode, and an electrolyte solution will be described with reference to an example of an embodiment.
In this embodiment, a "sodium secondary battery" is an electrochemical device in which charging and discharging occurs by the insertion and removal of sodium ions (Na ⁺ ), and is synonymous with a sodium secondary battery, a sodium ion secondary battery, a sodium ion battery, a sodium storage battery, a Na secondary battery, a Na ion battery, or a Na storage battery.
A "nonaqueous electrolyte" is an electrolyte that contains a nonaqueous solvent as a solvent, and an "aqueous electrolyte" is an electrolyte that contains an aqueous solvent as a solvent.
A "nonaqueous sodium secondary battery" is a sodium secondary battery that has a nonaqueous electrolyte as the electrolyte, and an "aqueous sodium secondary battery" is a sodium secondary battery that has an aqueous electrolyte as the electrolyte.

<Negative Electrode>
The negative electrode may include a negative electrode mixture containing a negative electrode active material containing the NaTP salt of this embodiment, and a current collector.
The negative electrode mixture contains a negative electrode active material, a binder, a conductive material, and, if necessary, an additive. The binder, the conductive material, and the additive may each be a known material.
The binder may be one or more selected from the group consisting of fluororesin, polyethylene, polypropylene, SBR material, and imide material, and may further be one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and ethylene tetrafluoroethylene (ETFE).
The conductive material may be one or more selected from the group consisting of carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials such as polyphenylene derivatives. Preferred carbon materials include graphite, soft carbon, hard carbon, carbon black, ketjen black, acetylene black, activated carbon, carbon nanotubes, carbon fibers, and mesoporous carbon.
The negative electrode mixture may be produced by a known method, and may be produced by mixing a negative electrode active material, a binder, and a conductive material in a desired ratio.

<Positive electrode>
The positive electrode may include a positive electrode mixture containing a positive electrode active material, a current collector, and, if necessary, an additive.
The positive electrode mixture contains a positive electrode active material, a binder, a conductive material, and, if necessary, an additive.
The positive electrode active material may contain a material that does not prevent the insertion and desorption of sodium ions from the negative electrode active material, and examples of the material include at least one of a sodium-containing transition metal oxide, a sodium-containing polyanion compound, and a carbon-based material.
The binder and conductive material may be any known material, and may be the same as the binder and conductive material that can be used in the negative electrode mixture described above.
The positive electrode mixture may be produced by a known method, and may be produced by mixing a positive electrode active material, a binder, and a conductive material in a desired ratio.

<Electrolyte>
The electrolytic solution is either a non-aqueous electrolytic solution or an aqueous electrolytic solution, and is preferably an aqueous electrolytic solution.
The electrolyte is a sodium salt, and a soluble sodium salt is preferable. Preferred electrolytes include one or more selected from the group consisting of NaCl, _{Na2SO4 , NaNO3} , _NaClO4 , NaOH, and _Na2S . From the viewpoint of ease of handling, the electrolyte is preferably one or more selected from the group consisting of NaCl, _{Na2SO4 , NaNO3} , and _NaClO4 , and more preferably at least one of NaCl and _NaClO4 .
The electrolyte concentration in the electrolytic solution is not particularly limited, but from the viewpoint of increasing the energy density of the sodium secondary battery, it is preferable that the electrolyte concentration (sodium salt concentration) in the electrolytic solution is high, and an example of the sodium salt concentration is a concentration of 1 mol/kg (1 m) or more and not more than the saturated solubility.
The electrolyte may contain an additive. The additive is not particularly limited, but may be one or more selected from the group consisting of succinic acid, glutamic acid, maleic acid, citraconic acid, gluconic acid, itaconic acid, diglycol, cyclohexanedicarboxylic acid, cyclopentanetetracarboxylic acid, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, sulfolane, dimethyl sulfone, and N,N-dimethylmethanesulfonamide. The content of the additive may be, for example, 0.01% by mass or more and 10% by mass or less in terms of the mass ratio of the additive to the mass of the electrolyte.

<Other configurations>
Other components such as the current collectors of the positive and negative electrodes and the separator may be any known components used in sodium secondary batteries or lithium secondary batteries.
A sodium secondary battery having a negative electrode containing the NaTP salt of this embodiment exhibits high cycle stability compared to conventional sodium secondary batteries, particularly conventional aqueous sodium secondary batteries having Nasicon-type _NaTi2 ( _PO4 ) ₃ as the negative electrode active material.

Next, the present disclosure will be described with reference to examples. However, the present disclosure is not to be construed as being limited to these examples.

<Evaluation of crystallinity>
The crystal structure of the NaTP salt was identified using a desktop X-ray analyzer (device name: MiniFlex600/ASC-8, manufactured by Rigaku Corporation) under the following conditions.
Target: Cu
Output: 0.6kW (15mA-40kV)
Step scan: 0.02° (2θ/θ)
Measurement time: 0.05 seconds

<Composition Analysis>
The composition of the NaTP salt was determined by dissolving 50 mg of a sample in an aqueous solution containing 10 mL of 35% hydrochloric acid and 1 mL of 35% hydrogen peroxide, and then performing ICP analysis on the solution. The ICP analysis was performed using an ICP emission analyzer (instrument name: ICPS-8100, manufactured by Shimadzu Corporation).

Observation of the surface layer
The measurements were performed using a transmission electron microscope (instrument name: JEM-2100, manufactured by JEOL Ltd.) and a thermal field emission scanning electron microscope (instrument name: JSM-7600F, manufactured by JEOL Ltd.).

<Sodium secondary battery>
(Preparation of Positive Electrode)
Nickel hexacyanoferrate _Na2Ni ([Fe(CN) ₆ ] having a cubic crystal structure was used as the positive electrode active material. _Na2Ni ([Fe(CN) ₆ ] was synthesized in the following manner. That is, _2.69 g of 10 mmol of Ni( _OCOCH3 ) _2.4H2O was dissolved in 175 mL of _H2O and 25 mL of DMF (N,N-dimethylformamide, HCON( _CH3 ) ₂ ) to obtain a first solution. Meanwhile, 4.84 g of 10 mmol of _Na4 [Fe(CN) ₆ ] _.10H2O and 7 g of NaCl were dissolved in 175 mL of _H2O to obtain a second solution.
The first solution was slowly added to the second solution, and the mixture was stirred at room temperature for 72 hours to obtain a precipitate, which was collected by centrifugation, washed with methanol, and then dried in air to obtain nickel hexacyanoferrate ( _Na2Ni [Fe(CN) ₆ ]) with a cubic crystal structure.
The obtained _Na2Ni ([Fe(CN) ₆ ], acetylene black (hereinafter also referred to as "AB"), and polytetrafluoroethylene (hereinafter also referred to as "PTFE") were mixed in a mass ratio of 70:25:5, and then molded into a pellet having a diameter of 3 mm to form a positive electrode (positive electrode mixture).

(Preparation of negative electrode)
The negative electrode active material and AB were mixed in a mass ratio of 84:16 using a planetary ball mill at 400 rpm for 1 hour under an Ar atmosphere to obtain a mixture. The mixture was then subjected to carbothermal treatment in an Ar stream at 800° C. for 1 hour, and then mixed with PTFE in a mass ratio of 95:5, molded into pellets with a diameter of 2 mm, and used as the negative electrode (negative electrode mixture).

(Preparation of aqueous sodium secondary battery)
An aqueous sodium _secondary battery was fabricated that had a positive electrode mixture as the working electrode (electrode corresponding to the positive electrode), a negative electrode mixture as the counter electrode (electrode corresponding to the negative electrode), a silver chloride electrode (Ag/AgCl) as the reference electrode, and an aqueous _NaClO4 solution with an electrolyte concentration (NaClO4 concentration) of 17 m as the electrolyte.

(Synthesis of NaTP salt)
Example 1
A mixed solution was obtained by mixing 2 times the molar amount of citric acid and 2 times the molar amount of hydrogen peroxide (H ₂ O ₂ ) relative to _{Ti (} OCH ₂ CH ₂ CH ₂ CH ₃ ) ₄ with 40 mL of ethanol solution in which Ti( _OCH 2 CH 2 CH ₂ CH 3 ) ₄ was dissolved. 1.05 times the molar amount of Na ₂ CO ₃ (0.525 as the molar ratio of Na to Ti) and 1 times the molar amount of NH ₄ H _{2 PO 4} were added to Ti(OCH ₂ CH ₂ CH ₂ CH ₃ ) 4 in the obtained mixed solution, and the mixture was evaporated to dryness at 60° C. for 30 minutes while stirring at 500 rpm, and then at 80° C. for 1 to 2 hours. Thereafter, the solid obtained by heating in air at 350°C for 5 hours was pulverized and mixed, and then heated in air at 700°C for 12 hours to obtain _NaTi2 ( _PO4 ) ₃ having a surface layer containing _Na5Ti ( _PO4 ) ₂ , which was used as the NaTP salt of this example. The crystallite size of the NaTP salt of this example was 36 nm, and the mass ratio of the surface layer was 1 mass%.
Using a planetary ball mill, the obtained NaTP salt and AB were mixed for 1 hour at 400 rpm under an argon atmosphere so that the mass ratio was 84: 16. The obtained mixture was fired for 1 hour at 800°C in an argon stream to obtain a negative electrode active material containing NaTi ₂ (PO ₄ ) ₃ of a Nasicon type structure (Nasicon type NaTi ₂ (PO ₄ ) ₃ ) having a surface layer.

Example 2
Except for using 1.1 times the molar amount _{of Na2CO3} (0.55 as the molar ratio of Na to Ti), the _NaTi2 ( _PO4 ) ₃ with surface layer and the negative electrode active material were obtained by the same method as in Example 1. The crystallite size of the NaTP salt in this example was 38 nm, and the mass ratio of the surface layer was 2 mass%.

Example 3
The _NaTi2 ( _PO4 ) ₃ with a surface layer and a negative electrode active material were obtained by the same method as in Example 1, except that _Na2CO3 was used in an amount of 1.15 times by _mole (the molar ratio of Na to Ti was 0.575). The crystallite size of the NaTP salt in this example was 41 nm.

Example 4
The _NaTi2 ( _PO4 ) ₃ with a surface layer and a negative electrode active _material were obtained by the same method as in Example 1, except that _Na2CO3 was used in an amount of 1.2 times by mole (the molar ratio of Na to Ti was 0.6). The crystallite size of the NaTP salt in this example was 49 nm.

Example 5
The _NaTi2 ( _PO4 ) ₃ with a surface layer and a negative electrode active material were obtained by the same method as in Example 1, except that _Na2CO3 was used in an amount of 1.5 times by _mole (the molar ratio of Na to Ti was 0.75). The crystallite size of the NaTP salt in this example was 50 nm.

Example 6
The _NaTi2 ( _PO4 ) ₃ with a surface layer and _a negative electrode active material were obtained by the same method as in Example 1, except that _Na2CO3 was used in a twice molar amount (the molar ratio of Na to Ti was 1). The crystallite size of the NaTP salt in this example was 50 nm.

<Comparative Example 1>
Except for using 1 times the molar amount _{of Na2CO3} (0.5 as the molar ratio of Na to Ti), the same method as in Example 1 was used to obtain _NaTi2 ( _PO4 ) ₃ and a negative electrode active material with a Nasicon structure that does not have a surface layer containing _Na5Ti ( _PO4 ) ₃ . The crystallite size of the NaTP salt in this comparative example was 34nm.

The XRD patterns of the negative electrode active materials obtained in the examples and comparative examples are shown in Figures 1 and 2. In all cases, the XRD peak of _NaTi2 ( _PO4 ) ₃ was confirmed. In Examples 5 and 6, the XRD peaks attributed to _Na5Ti ( _PO4 ) ₃ were also confirmed, and it was found that in the examples, NaMP salts having a surface layer (coating layer) containing _Na5Ti ( _PO4 ) ₃ were obtained.
FIG. 3A shows a cross-sectional TEM observation diagram of NaTi ₂ (PO ₄ ) ₃ of Example 2. FIG. 3B shows a cross-sectional TEM observation diagram of NaTi ₂ (PO ₄ ) ₃ of Comparative Example 1. In NaTi ₂ (PO ₄ ) ₃ of Example 2, NaTi ₂ (PO ₄ ) ₃ is white, the surface layer containing Na ₅ Ti (PO ₄ ) ₃ is gray, and the outside of the particle (background) is black (see FIG. 3A). From this, it was confirmed that there is a surface layer of 2 to 7 nm, or even 5 nm, between NaTi ₂ (PO ₄ ) ₃ and the particle. In contrast, in NaTi ₂ (PO ₄ ) ₃ of Comparative Example 1, NaTi ₂ (PO ₄ ) ₃ is gray, and the outside of the particle (background) is black (see FIG. 3B). 3A and 3B, it was confirmed that the surface structures of the particles of NaTi ₂ (PO ₄ ) ₃ in Example 2 and Comparative Example 1 are different, and that the NaTi ₂ (PO ₄ ) ₃ in Example 2 has a surface layer on the surface of the particles.

<Measurement Example 1> (Charge-discharge cycle test)
Using an aqueous sodium secondary battery equipped with the negative electrode active material of the Example or Comparative Example, charging and discharging were repeated under the following conditions, and the discharge capacity of each charge/discharge cycle was measured. The ratio (%) of the discharge capacity at the 30th cycle to the discharge capacity at the 1st cycle was taken as the capacity retention rate, and the output characteristics were evaluated based on this.
Current density: (1 to 5 cycles) 2mA/ ^cm2
(6 to 10 cycles) 5mA/ ^cm2
(11 to 15 cycles) 10mA/ ^cm2
(16 to 20 cycles) 15mA/ ^cm2
(21 to 25 cycles) 20mA/ ^cm2
(26 to 30 cycles) 25mA/ ^cm2
(31 to 35 cycles) 2mA/ ^cm2
Voltage: -0.9V to -0.3V (vs Ag/AgCl reference electrode)
Charge/discharge temperature: 25°C
The discharge capacities from cycles 1 to 35 are shown in Figure 4. The capacity retention rates are shown in Table 1 below.

図４より、２０ｍＡ／ｃｍ^２を超える高出力放電では、表面層（被覆層）を有するＮａＴＰ塩の充放電容量が大きく、表面層の組成に影響すると考えられるＮａ_２ＣＯ_３量が少ない実施例ほどこの傾向が顕著になった。
なお、３０サイクル後、放電電流密度を２ｍＡ／ｃｍ^２として充放電した結果、実施例及び比較例とも同様な放電容量であった。これより、表面層はサイクル特性への影響は小さく、高出力放電、すなわち高電流密度における充放電時の容量低下を抑制していることが確認できた。

As shown in FIG. 4, in high-output discharge exceeding 20 mA/ ^cm2 , the charge/discharge capacity of the NaTP salt having a surface layer (coating layer) was large, and this tendency became more pronounced in the examples having a _smaller amount of _Na2CO3 , which is thought to affect the composition of the surface layer.
After 30 cycles, the battery was charged and discharged at a discharge current density of 2 mA/ ^cm2 , and the discharge capacity was similar in both the Example and the Comparative Example. This confirmed that the surface layer had little effect on the cycle characteristics and suppressed the capacity decrease during high-output discharge, i.e., charging and discharging at a high current density.

This application claims priority based on Japanese patent application No. 2021-39890, filed on March 12, 2021, and incorporates by reference all of the contents of that Japanese patent application.

Claims (9)

It is represented by the general formula _{Na1+ xTi2} ( _PO4 ) ₃ (where 0≦x≦2) , characterized in that it has a surface layer containing _Na5Ti ( _PO4 ) ₃ ;
Sodium titanium phosphate _, wherein the surface layer is more than 0 mass % and less than 5 mass % of sodium titanium phosphate represented by the general formula Na1 _+xTi2 ( PO4 ₎ 3 ₍ where 0≦x≦2) . The sodium titanium phosphate according to claim 1, wherein the crystal structure of the sodium titanium phosphate is a Nasicon type crystal structure. 3. The sodium titanium phosphate according to claim 1 or 2, wherein the surface layer is made of Na ₅ Ti(PO ₄ ) ₃ having a Nasicon type crystal structure . The sodium titanium phosphate according to any one of claims 1 to 3, wherein the thickness of the surface layer is greater than 0 nm and less than or equal to 50 nm. The sodium titanium phosphate according to any one of claims 1 to 4, having a crystallite size of 10 nm or more and 100 nm or less. The method for producing sodium titanium phosphate according to any one of claims 1 to 5, comprising the steps of calcining a mixture containing a sodium source, a phosphate source, and a titanium source at 150°C to 400°C to obtain a primary calcined product, crushing the primary calcined product, and calcining it at 400°C to 900°C. A negative electrode active material containing the sodium titanium phosphate according to any one of claims 1 to 5. A sodium secondary battery comprising a negative electrode containing the sodium titanium phosphate according to any one of claims 1 to 5, a positive electrode and an electrolyte. The sodium secondary battery according to claim 8, wherein the electrolyte is an aqueous electrolyte.